The present invention relates to a torque distribution control system for a four-wheel drive motor vehicle having a central differential, and more particularly to a control system in which output torque of a transmission is unequally distributed to front wheels and rear wheels in accordance with driving conditions.
In a full-time four-wheel drive motor vehicle with a central differential, a fluid-operated multiple-disk friction clutch is provided for restricting the differential operation. The torque distributed to the front wheels and the rear wheels is controlled by changing the torque of the friction clutch, thereby improving straight-ahead stability, acceleration and driveability. In order to widely control the torque distribution, it is necessary for the central differential to be arranged to unequally distribute the torque to the front wheels and the rear wheels. There are two methods for the unequal distributing system. One is a front-weighed distribution which is weighed to the front wheels, and the other is a rear-weighed distribution which is weighed to the rear wheels. In the front-wheel weighed system, larger torque is transmitted to the front wheels. Accordingly, although the vehicle has good straight-ahead stability, the steerability deteriorates because of drifting out of the course. In the rear-wheel weighed system, the larger torque is transmitted to the rear wheels. The vehicle has good steerability and driveability on a dry road. However, the vehicle is liable to spin on a slippery road.
On the other hand, a disadvantage of the four-wheel drive vehicle is that all of the four wheels may slip (slipping spin) at the same time, causing difficulty in driving.
In order to ensure driving stability of the vehicle, the torque to the rear wheels is set to a larger value than that to the front wheels, so that the rear wheels may slip first. Thus, the vehicle can be safely driven by the front wheels at a small torque while the rear wheels idle.
Since the driving force (acceleration force) is distributed to the four wheels, each wheel can sustain a larger side force. Thus, in order to ensure driving stability in the rear-wheel weighed system, it is necessary to keep the sustainable side force of the rear wheels large.
The driving force and the side force change with the slip ratio of the wheel to the ground (slip ratio is described hereinafter in detail). The slip ratio changes in accordance with the condition of the road surface and the coefficient of friction of the tire. In particular, if the slip ratio is larger than a predetermined value (10 to 20%), the driving force and the sustainable side force remarkably reduce, which results in a deterioration of driving stability. Accordingly, the slip ratio of the rear wheel is controlled so as not to exceed the predetermined value. If the system operates to increase the torque distributed to the front wheels in accordance with the reduction of the sustainable side force, slipping of the rear wheels is prevented to ensure the driving force. Therefore, it is desirable to control the torque distribution based on the rear-wheel slip ratio.
Japanese Patent Applications Laid-Open 62-55228, 62-261539 and 63-8026 disclose four-wheel drive control systems in which a central differential comprising a simple planetary gear device is provided. Standard torque to the front and rear wheels is unequally distributed at a ratio determined by the difference between pitch circles of gears of the planetary gear device. Torque capacity of the clutch is controlled in accordance with the difference between speeds of the front wheels and the rear wheels, or with a predetermined value corrected by vehicle speed and steering angle.
Further, Japanese Patent Application Laid-Open 63-170129 discloses a control system in which the torque distribution is controlled in accordance with a drive wheel slip ratio. The torque distributed to the front wheels and the rear wheels is controlled so as to cause the drive wheel slip ratio to coincide with desired slip ratio for obtaining a large driving power of the wheels.
However, in the conventional systems, the standard torque distribution ratio determined by the ratio of the pitch circles can not be changed, unless the diameters of the gears change. However, it is difficult to change the diameters because of a limitation of space. Since the distribution of the torque to the rear wheels can not be set to a large value, the control range of the torque distribution becomes small.
Further, the torque distribution is controlled by restricting the differential operation of the central differential. Therefore, slip conditions of the wheels on the road surface can not be detected so that it is impossible to actively control the torque distribution in accordance with the slip conditions. In addition, on a road surface with a low coefficient of friction, if the vehicle is driven under slipping conditions and the speed difference is small, the torque distribution can not be controlled.
In the fourth conventional system, the slip ratio is controlled in accordance with .mu.-s characteristic (s: slip ratio) to obtain the desired slip ratio of 10 to 20% of providing a maximum driving force. The system aims at only improving driveability without considering driving stability, and hence it is not preferable to use such a system in an ordinary motor vehicle.